1. Field of the Invention
The present invention relates to a wire-tap preventing apparatus for community antenna television terminals in a chargeable or pay television system and the like.
2. Description of the Prior Art
In a chargeable television system to which are connected a plurality of subscribers' terminals, hereinafter referred to simply as OWAs, a viewing fee is set for each television channel, and, when a viewer wishes to view a program on a certain television channel, the viewer is charged the fee set for that television channel.
Data for permitting the viewing of the television channel is transmitted from a community antenna television center for comparison with the address of the OWA in the subscribing viewer's home, and if there is a favorable comparison, the subscriber is permitted to view the program on the channel. In a community antenna television system, hereinafter referred to simply as CATV, all of the subscriber addresses are made different in order to distinguish the subscribers from each other.
Referring to FIG. 1 there is shown a conventional OWA of the one-way type, in which the down polling data corresponds to the subscriber's terminal address and view-controlling data. The conventional OWA consists of an FSK receiver 1, a converter 2, a phase-locked loop unit 3, hereinafter referred to simply as PLL unit, a control unit 4, keys 5 for allowing a subscriber to select a desired television channel, and an address section 6. In the control unit 4, the output signal from the FSK receiver 1 is applied not only to the polling address data register 4a but also through a switch SW1 to RAM 4b which stores viewing-control data, and the address data inputted from the address portion 6 into the control unit 4 is applied to a comparator 4d through an address input register 4c in order to compare the address data with the data from the polling address register 4a; as a result, an output signal is produced by the comparator 4d to operate the switch SW1 to effect ON/OFF control thereof. When the subscriber, or viewer, selects one television channel by operating the keys 5 for the television-channel selection, the channel address is stored in a channel register 4e.
Referring to the operation of the controller 4f, the controller 4f provides access for loading the contents of the address section 6 into the address input register 4c, and also generates an output signal for controlling a switch SW2 on the basis of the contents (viewing-control data) stored in the RAM 4b and the contents stored in the channel register 4e, and, according to the ON/OFF state of the switch SW2, the path of the data from the PLL data memory 4g is controlled either to permit or to inhibit viewing. According to this CATV system, both the picture image of the program on a television channel and also the polling data are transmitted from the CATV center, and at the subscriber's terminal the picture and the polling data are received. Then, the polling data is compared with the subscriber's particular pre-assigned address data, which is different from the other subscriber's addresses within the CATV system. When the comparison produces a coincidence, the down data is fetched to use it for permitting or inhibiting the viewing of the television channel. This address data is inputted to the control unit 4 and serves as a reference for detecting coincidence therewith of the polling data. The switch SW1 in FIG. 1 is turned OFF when the polling address data is not coincident with the terminal's set address data, and is turned ON upon coincidence thereof. Further, the switch SW2 is turned ON when the RAM 4b (containing viewing-control data corresponding to the channel register) is in a viewing-permitting mode, and is turned OFF in the viewing-inhibiting mode. When the switch SW2 is ON, the data from the PLL data memory 4g is stored in the PLL unit 3 to display the picture image of the selected television channel on the television set in the subscriber's home.
The address section 6 of the terminal is composed of, for example, a jumper and a DIP switch which are easily discernible from the outside of the terminal unit, so that it is easy for an unauthorized person to change or modify the address from the pre-assigned set address to another address by changing, for example, the setting of the DIP switch. On the other hand, the control unit 4 cannot detect such changing of the address, and further the OWA system has no up-data; therefore, the CATV center cannot detect such unauthorized changing of the address occurring in the subscriber's terminal, which means that a subscriber can purloin viewing of television programs merely by changing the address, thereby making impossible the proper management of the CATV system.